Treat subdomains of the site host as internal links

CHANGELOG.md

@@ -1,5 +1,6 @@
 # main [(unreleased)](https://github.com/fastruby/jekyll-external-link-accessibility/compare/v0.2.0...main)
 
+- [FEATURE: Treat subdomains of the site host as internal links, so they keep their link equity](https://github.com/fastruby/jekyll-external-link-accessibility/pull/7)
 - [CHORE: Add an RSpec test suite and CI matrix running Ruby 2.7 through 4.0](https://github.com/fastruby/jekyll-external-link-accessibility/pull/6)
 
 # v0.2.0 / 2026-07-15 [(commits)](https://github.com/fastruby/jekyll-external-link-accessibility/compare/v0.1.0...v0.2.0)

Gemfile.lock

@@ -1,7 +1,7 @@
 PATH
   remote: .
   specs:
-    jekyll-external-link-accessibility (0.2.0)
+    jekyll-external-link-accessibility (0.3.0)
       jekyll (~> 4.0, >= 4.0.1)
       nokogiri (>= 1.12.0, < 2.0)
 

README.md

@@ -19,7 +19,7 @@ This plugin makes every link in your blog posts open in a new tab accessibly (`t
 The plugin edits every link in a post, internal and external:
 
 - All links open in a new tab so readers don't lose their place, with a `title`, a new-tab icon, and a screen-reader-only "opens a new window" note. Skip a specific link by adding `data-no-external="true"`, e.g. `<a href="...." data-no-external="true">...</a>`.
-- External links (pointing to a host other than your site's `url` in `_config.yml`) also get a `rel` attribute (`external nofollow noopener noreferrer` by default, see Configuration). The `nofollow` keeps them from passing your link equity off-site, so internal links are left without a `rel`. The `www.` prefix and host casing are ignored when comparing hosts.
+- External links (pointing to a host other than your site's `url` in `_config.yml`) also get a `rel` attribute (`external nofollow noopener noreferrer` by default, see Configuration). The `nofollow` keeps them from passing your link equity off-site, so internal links are left without a `rel`. The `www.` prefix and host casing are ignored when comparing hosts, and subdomains of your site's host (e.g. `docs.example.com` when your `url` is `https://example.com`) count as internal.
 
 ### Configuration
 You can override the default configuration by adding the following section to your Jekyll site's `_config.yml`:

lib/jekyll-external-link-accessibility.rb

@@ -40,13 +40,14 @@ def self.modify_links(page)
     end
 
     # A link is external only when it points to a different host than the site.
-    # Relative links ("/blog/...") and absolute links to our own domain are internal,
-    # so they keep their link equity (no nofollow).
+    # Relative links ("/blog/..."), absolute links to our own domain and links to a
+    # subdomain of it are internal, so they keep their link equity (no nofollow).
     def self.external_link?(href, site_host)
       return false unless href.start_with?(*EXTERNAL_SCHEMES)
 
       link_host = host_for(href)
       return true if link_host.nil?
+      return false if site_host && link_host.end_with?(".#{site_host}")
 
       link_host != site_host
     end

lib/jekyll-external-link-accessibility/version.rb

@@ -2,6 +2,6 @@
 
 module Jekyll
   class ExternalLinkAccessibility
-    VERSION = '0.2.0'
+    VERSION = '0.3.0'
   end
 end

spec/jekyll-external-link-accessibility_spec.rb

@@ -43,6 +43,21 @@ def rewrite(inner_html, config: { 'url' => 'https://example.com' }, wrapper: 'po
       expect(described_class.external_link?('https://www.example.com/blog', site_host)).to be(false)
     end
 
+    it 'is false for a subdomain of the site host' do
+      expect(described_class.external_link?('https://docs.example.com/x', site_host)).to be(false)
+      expect(described_class.external_link?('https://www.docs.example.com/x', site_host)).to be(false)
+      expect(described_class.external_link?('//docs.example.com/x', site_host)).to be(false)
+    end
+
+    it 'is true for hosts that only look like a subdomain' do
+      expect(described_class.external_link?('https://notexample.com', site_host)).to be(true)
+      expect(described_class.external_link?('https://example.github.io', site_host)).to be(true)
+    end
+
+    it 'is true for any host when the site has no url configured' do
+      expect(described_class.external_link?('https://other.com', nil)).to be(true)
+    end
+
     it 'is false for relative and anchor links' do
       expect(described_class.external_link?('/blog/post', site_host)).to be(false)
       expect(described_class.external_link?('#section', site_host)).to be(false)
@@ -88,6 +103,13 @@ def rewrite(inner_html, config: { 'url' => 'https://example.com' }, wrapper: 'po
       expect(internal['rel']).to be_nil
     end
 
+    it 'does not add rel to a subdomain of the site host' do
+      link = rewrite("<a href='https://docs.example.com/x'>x</a>").at_css('a')
+
+      expect(link['rel']).to be_nil
+      expect(link['target']).to eq('_blank')
+    end
+
     it 'skips links with no href, empty href, anchors or data-no-external' do
       doc = rewrite(<<~HTML)
         <a>no href</a>